In the recent years, many of the tasks which the pilot of an aircraft used to perform manually, are being performed by an on board system according to the direction of gaze of the pilot. For example, a camera mounted to the head of the pilot can register a target according to the gaze of the pilot. However, since the camera is located behind the canopy of the aircraft, the transparent canopy material deviates the light beam which is transmitted from the target to the camera.
This deviation however small, can have deleterious effects when the target is located far away. According to document AFAMRL-TR-81-21 described herein below, for each milliradian (mr) of error, the true position of the target is displaced from the apparent position thereof, by one foot for each 1000 feet of range. For example, a canopy which induces 10 mr of error, can move the apparent position of a target located 3000 feet away, a distance of 30 feet, more than enough to miss the target. Systems and methods for measuring the distortion caused by the canopy and for correcting for this distortion are known in the art.
U.S. Pat. No. 4,377,341 issued to Task et al., and entitled “System for Measuring Angular Deviation in a Transparency”, is a directed to a system for measuring the angular deviation caused by an aircraft windscreen, according to the readings of a first linear detector device and a second linear detector device. The system includes an incandescent lamp, a condensing lens, a target slide, a projection lens, a receiving lens, a beam splitter, the first linear detector device and the second linear detector device.
The condensing lens is located between the incandescent lamp and the target slide. The projection lens is located between the target slide and the aircraft windscreen. The aircraft windscreen is located between the projection lens and the receiving lens. The target slide is an opaque material which includes a transparent “L” pattern, having a pair of transversely aligned linear segments. The projection lens collimates the image of the “L” pattern illuminated by the incandescent lamp and directs it toward the aircraft windscreen. The receiving lens collects the light transmitted by the aircraft windscreen, and directs it toward the beam splitter. The beam splitter divides the light to a first channel toward the first linear detector device and to a second channel toward the second linear detector device. In this manner, the first linear detector device detects a first segment of the pair of transversely aligned linear segments, and the second linear detector device detects a second segment of the pair of transversely aligned linear segments. The angular deviation is determined according to the relative locations of the first segment and the second segment, on the first linear detector device and the second linear detector device, respectively.
U.S. Pat. No. 4,461,570 issued to Task et al., and entitled “Method for Dynamically Recording Distortion in a Transparency”, is directed to a system for making a qualitative photographic record of the degree of distortion of a transparency at a plurality of regions thereof. The system includes a support fixture, a test target, a controller, and a camera. The support fixture mounts to the transparency. The lens of the camera is located at a design eye behind the transparency, at a point where an occupant of a vehicle to which the transparency is attached, is stationed. The camera and the support fixture are fixed to the ground. A first drive means attached to the support fixture move the transparency relative to the camera, about a vertical axis, and a second drive means move the transparency about a horizontal axis. The controller is connected with the first drive means and the second drive means. The lens of the camera is located at the intersection of the vertical axis and the horizontal axis.
The test target includes a plurality of light sources arranged in a rectangular dot matrix target pattern. The test target is located in front of the transparency. The photographic recording is performed in a dark room. As the transparency is moved about a predetermined one of the axes through a predetermined angle or distance, a shutter of the camera is left open for the duration of the movement. If no distortion is present, the photographic record shows only the rectilinear array of the light sources. However, if distortion is present, then the photographic record shows distortion tracks. The distortion tracks indicate the varied angular deviations suffered by the light rays at different regions of the transparency.
U.S. Pat. No. 5,302,964 issued to Lewins and entitled “Heads-Up Display (HUD) Incorporating Cathode-Ray Tube Image Generator with Digital Look-Up Table for Distortion Correction”, is directed to a HUD for displaying an image on a cathode-ray tube (CRT), by reversing the inherent distortion in the HUD. The HUD includes an electronic unit, the CRT, a combiner, a vector signal generator, a digital signal corrector, a horizontal digital to analog converter (DAC), and a vertical DAC. The electronic unit includes a radar unit, a plurality of sensors and a computer. The vertical signal generator includes a raster image generator section and a stroke image generator section. The digital signal corrector includes a horizontal axis look-up table memory and a vertical axis look-up table memory.
The vector signal generator is connected with the electronic unit and with the digital signal corrector. The vertical DAC and the horizontal DAC are connected with the digital signal corrector and with the CRT. The data generated by the radar unit and the sensors are fed to the computer. The computer feeds data to the vertical signal generator. The vertical signal generator generates a digital horizontal signal and a digital vertical signal, and feeds them to the digital signal corrector. The digital signal corrector generates a digital horizontal deflection signal and a digital vertical deflection signal, which are corrected for inherent distortions in the HUD. The horizontal DAC and the vertical DAC generate a horizontal analog deflection signal and a vertical analog deflection signal, respectively, according to the digital horizontal deflection signal and the digital vertical deflection signal, respectively.
The computer is programmed to alternately switch between a radar raster image and an information stroke image, such that both appear on the combiner, superimposed on a view through an aircraft windshield. The digital signal corrector is configured such that an image on the combiner as viewed from a predetermined angle, will appear undistorted. The digital signal corrector predistorts the digital horizontal deflection signal and the digital vertical deflection signal, in a manner which is the inverse of the inherent distortion in the HUD, such that the image on the combiner appears the same the one which would be generated according to the digital horizontal signal and digital vertical signal.
Document AFAMRL-TR-81-21 of the US Air Force Aerospace Medical Research Laboratory, by Louis V. Genco and Harry L. Task, February 1981, and entitled “Aircraft Transparency Optical Quality: New Methods of Measurement”, is directed to methods and systems for measuring the distortion of an image transmitted by a transparency. The document describes methods for determining the distortion in a transparency, by analyzing grid board photographs. The grid board photograph is analyzed by determining the slope of grid lines, by measuring the lens factor or by digital processing.
In the grid line slope method, the enlarged photograph is fixed to a drafting board and aligned, such that the undistorted grid lines are horizontal. The slope of the lines photographed through the transparency are determined by drafting instruments, for several horizontal and vertical lines. In the lens factor method, a baseline measure is made of a baseline count of grid squares per inch, in an undistorted portion of the picture. Several transparency counts of grid squares per inch, are made for the grid areas photographed through the transparency. The most deviant of the transparency counts is compared with the baseline counts, by dividing the larger number by the smaller one.
In the digital processing method, an electronic digitizer is employed to sample salient portions of the photograph. The technician places a “bug” consisting of a cross hair under a magnifying lens, on a series of grid line intersections. When alignment has been achieved, he presses a button to record the position of the “bug”. The digitized data are processed by a computer to yield lens factor and grid line slope without using drafting instruments.
The grid board photographs are made either by single exposure method or multiple exposure method. In the single exposure method, a photograph of a specified grid board is made with the transparency, camera and grid board at specified distances. A portion of the field of view includes an area of the grid board outside the area of the transparency. The distortion appears on the final photograph, as a bending or blurring of the grid lines. In the multiple exposure method, one exposure is made without the transparency in place and one with. The final photograph shows a plurality of lines, one set being undistorted and used as a reference, while the other set contains the distorted image.